| Geothermal energy is a new type of clean and environment-friendly renewable energy with large reserves and wide distribution.It is of great significance for China to adjust its energy structure,improve the environment and realize green and low-carbon development.Foam drilling has the advantages of safety,high efficiency and underbalanced operation.It has been widely used in the development of high-temperature and high-pressure geothermal resources.It is of great theoretical value and engineering guiding significance to study the wellbore flow and heat transfer and the law of annulus rock carrying in the process of foam drilling in geothermal Wells.Based on relevant theoretical knowledge of drilling fluid rheology,computational fluid mechanics,heat transfer,etc.,this paper conducts in-depth research on the rheological change rule of foam drilling fluid under high temperature and high pressure,the distribution rule of wellbore temperature and pressure during the foam drilling cycle,the efficiency of foam rock carrying and the design of drilling parameters.The experiment of density and rheological changes of foam drilling fluid under high temperature and high pressure was carried out,and the specific effects of temperature and pressure on foam density and rheology were analyzed.Through binary nonlinear fitting of experimental data,a dynamic calculation model of density and rheological properties about temperature and pressure is established,which is closer to the true properties of drilling fluid at different well depths in the wellbore,improving the iteration of the transient annulus rock-carrying model below The accuracy of the calculation.The transient flow model of rock carrying in the annulus of geothermal foam drilling was established,and the mass conservation equation,momentum conservation equation and energy conservation equation were constructed according to the characteristics of geothermal reservoir fissure height development,the influence of factors such as the loss of drilling fluid and high temperature in the geothermal drilling process.The finite difference method is used to discretize the mathematical model,and the Gauss-Seidel method is used to iteratively solve the system of equations.The process and calculation results of wellbore temperature,pressure and cuttings concentration are given.The numerical simulation study on the flow heat transfer and annulus rock-carrying law of high-temperature geothermal foam drilling has been carried out.The increase of liquid injection flow rate and gas injection flow rate can help reduce the temperature of the wellbore and the concentration of annulus rock cuttings,and improve the drilling efficiency.Increasing the wellhead back pressure,foam loss,and ROP parameters will reduce the annulus rock-carrying efficiency and increase the concentration of cuttings.The reasons for the corresponding changes are given,and the flow and heat transfer law,the rock-carrying efficiency change law and the annulus pressure drop change law of the foam drilling fluid during the geothermal well drilling process are revealed.In this paper,the density and rheological change rule experiment of foam drilling fluid under high temperature and high pressure was designed and carried out,and the transient model of flow heat transfer and rock carrying during geothermal foam drilling was established.The change rule of wellbore temperature,pressure and annulus cuttings concentration was obtained,which can provide support for the development of middle and deep geothermal resources. |
